Project summary Bone mass is determined by the balance between bone formation by osteoblasts and bone resorption by osteoclasts. Osteocytes, post-mitotic cells embedded deeply within bone, control this balance by secreting paracrine factors that control osteoblast and osteoclast activity. Therefore, therapies that target osteocytes represent promising new strategies to treat post-menopausal osteoporosis, a common and serious public health problem in our aging population. Currently, the only approved osteoporosis treatment that stimulates new bone formation is teriparatide, amino acids 1-34 of parathyroid hormone (PTH). Osteocytes respond to external cues, such as PTH, to orchestrate bone remodeling. A crucial step in the signaling cascade through which osteocytes respond to PTH is inhibition of the kinase salt inducible kinase 2 (SIK2). Small molecule SIK2 inhibitors, such as YKL-05-099, mimic PTH action, both in cultured osteocytes in vitro and in vivo. YKL-05-099 increases bone mass and bone formation by osteoblasts in young, eugonadal mice. Surprisingly, although YKL-05-099 increases levels of the key osteoclastogenic cytokine RANKL, this compound actually reduces osteoclast numbers in vivo. Therefore, YKL-05-099 displays the promising therapeutic combination of both increasing osteoblast activity and reducing osteoclast activity in vivo. However, the mechanism whereby YKL-05-099 inhibits osteoclasts is not known. Aim 1 of this proposal will explore effects of YKL-05-099 on osteoclasts. Unbiased kinase profiling revealed that, in addition to SIK2, the M-CSF receptor is a potential target of YKL-05-099. Here we will test the hypothesis that YKL-05-099-induced inhibition of osteoclastogenesis involves inhibiting signaling steps downstream of the M-CSF receptor. While YKL-05-099 increases bone mass in young, eugonadal mice, its in vivo properties in older, hypogonadal animals remains unknown. Post-menopausal osteoporosis is characterized by accelerated bone resorption and a concomitant insufficient osteoblastic response. Based on its cellular mechanism of action, we predict that YKL-05-099 will restore bone mass in a rodent model of female hypogonadal osteoporosis. Aim 2 of this proposal will test the skeletal effects of YKL-05-099 treatment in female mice who have undergone surgical ovariectomy. Bone mass will be determined by micro-CT, bone strength by biomechanical testing, and cellular composition activity by static and dynamic histomorphometry. Taken together, these studies will clarify the mechanism of action through which YKL-05-099 affects bone biology, and will determine whether this small molecule restores bone loss in a model of post-menopausal osteoporosis.